The present invention relates to an alkaline battery which uses mercuryless zinc powder as a negative electrode active material, and uses silver oxide, manganese dioxide, oxygen or the like as a positive electrode active material, and a clock or watch which uses the alkaline battery.
Conventional alkaline batteries suffer from the drawbacks in that zinc powder used in the alkaline battery is corroded and dissolved by an alkaline electrolyte solution, and in that the generation of hydrogen gas and the self-discharge in battery performance accompanying therewith are large. In addition, a battery is formed by a collector such as copper or the like which contacts with zinc, and hydrogen gas has been also generated therefrom. In the prior art, as a countermeasure to prevent the above, zinc is mercurated with mercury having a high hydrogen overvoltage, or zinc oxide is added to an electrolyte solution up to approximate saturation.
However, environmental pollution due to mercury from used dry batteries has become a problem in recent years, and various studies for reducing mercury have been performed. Among processes undertaken during the studies are the formation zinc alloys, plating of a collector, and addition of an organic or inorganic inhibitor to an electrolyte solution.
The formation of zinc alloys has been performed for a fairly long time, and metals such as bismuth, indium, lead and the like have been investigated. Many patent applications pertaining to this process have been also filed, for example, Japanese Patent Publication No. 25-27822 (1950), Japanese Patent Publication No. 33-3204 (1958), Japanese Patent Publication No. 63-3942 (1988), and Japanese Patent Application Laid-Open No. 1-10861 (1989).
As the inorganic inhibitor, indium oxide and indium hydroxide as the indium compound have been frequently studied, and many patent applications have been also filed, including for example, Japanese Patent Publication No. 51-36450 (1976), Japanese Patent Application Laid-Open No. 49-93831 (1974), Japanese Patent Application Laid-Open No. 49-112125 (1974), Japanese Patent Application Laid-Open No. 59-186255, Japanese Patent Application Laid-Open No. 59-186256 (1984), and Japanese Patent Application Laid-open No. 4-26061 (1992). The use of a compound of alkaline earth metal as the inorganic inhibitor is addressed in Japanese Patent Application Laid-Open No. 49-8727 (1974), Japanese Patent Application Laid-Open No. 49-93831 (1974), and Japanese Patent Application Laid-Open No. 49-121926 (1974). The use of organic inhibitors is addressed in Japanese Patent Application Laid-Open No. 2-86064 (1990), and Japanese Patent Application Laid-Open No. 3-29270 (1991).
On the other hand, with respect to the process of plating a collector, the surface has been coated with indium or tin having a high hydrogen overvoltage by means of a method of plating or the like, and the formation of a battery due to the contact with zinc is prevented so as to suppress the generation of hydrogen. Such process is addressed in Japanese Patent Application Laid-Open No. 52-74834 (1977), Japanese Patent Application Laid-Open No. 52-98929, Japanese Patent Application Laid-Open No. 60-221958 (1985), and Japanese Patent Publication No. 52-42211 (1977).
In the prior art, investigation has been made as to each individual technique as described above, however, because the strong anticorrosion agent mercury has been used, optimization by combination of the characteristics of each technique has not been frequently performed.
Mercury, which is added in order to prevent corrosion and dissolution of zinc, is not only expensive from a viewpoint of cost, but is also associated with the problem of environmental pollution. In addition, the addition of zinc oxide also includes such a task that the viscosity of the electrolyte is raised, and the conductivity is lowered.
Indium oxide and indium hydroxide as the inorganic inhibitor also includes many problems.
Indium oxide is extremely difficult to dissolve in the electrolyte solution which is a caustic alkali, and hydrogen gas is consequently generated due to the contact of indium oxide with zinc powder or the collector. This is considered to be due to the fact that the solubility of indium oxide is poor, it is impossible to supply indium ion of a degree to sufficiently coat the zinc surface and the collector surface, and indium oxide becomes conductive due to inevitable impurities during production contacts with zinc and the collector, resulting in formation of a local battery.
It is said that as compared with indium oxide, indium hydroxide to some extent dissolves in an electrolyte solution of caustic alkali, and its solubility relates to the size and crystallinity of particles. However, as compared with indium compounds such as indium sulfate, indium sulfamate, indium chloride and the like, it is extremely difficult to dissolve. Thus, the same problems are associated as with of indium oxide. In addition, indium as an amphoteric compound generates polyion together with hydroxide ion (those similar to the description in Inorganic Chemistry Series 7, Coordinate Stereochemistry, written by Yoichi NIIMURA, published by Baihukan Co. Ltd., 65-66), and increases the viscosity of the electrolyte solution, so that it lowers the conductivity of the electrolyte solution and deteriorates battery performance.
Considerable advantageous effects are appreciated in the use of the indium compound as the inhibitor which is easier to dissolve in the electrolyte solution as compared with the case in which the conventional scarcely soluble inhibitor is used. However, in order to further utilize the characteristic of the indium compound, it is necessary to also solve problems as follows.
The electrode potential of zinc is lower than the deposition potential of indium, so that when indium ion is present in the electrolyte, indium is deposited as metal on zinc and the collector contacting with zinc. However, hydrogen generation is accompanied as a competitive reaction in accordance with the deposition reaction of indium, and this has been the cause of deficiencies such as liquid leakage and expansion of the alkaline battery. In addition, there has been such a problem that indium ion which is not deposited precipitates as hydroxide and decreases the conductivity of the electrolyte solution.
Other than the indium compound, compounds of metals having a relatively high hydrogen overvoltage such as tin and lead are used as the inhibitor, however, there have been problems as follows.
Metal ion, which is supplied from the metal compounds of these metals to the electrolyte solution, is reduced on the surface of zinc and the collector, and deposited as metal. However, when the surface is coated with one species of metal, crystal grains become coarse, it is impossible to homogeneously coat the surface, and the effect is reduced. It is difficult to suppress the hydrogen generation and improve the discharge characteristic by means of a single metal. In addition, the compounds of indium and the like are expensive, so that the use of only one species becomes expensive from a viewpoint of cost.
With respect to the corrosion and dissolution of zinc, there is considered a case in which zinc itself is corroded by water and the hydroxyl group in the alkaline solution, and a case in which a local battery is formed by the contact with metals such as copper, brass and the like of the collector which are nobler than zinc resulting in dissolution. Thus, attempts have been frequently made to add a metal having a high hydrogen overvoltage to zinc to form an alloy so as to suppress corrosion and dissolution. It is known that the effect thereof is remarkably expressed especially when indium is added. When zinc is used in which indium is added in a relatively high concentration by, for example, not less than 400 ppm, a part of indium and zinc is once dissolved by the contact with copper and the like of the collector. It is considered that the corrosion and dissolution of zinc are suppressed by a mechanism that the dissolved indium ion is then deposited on the collector, and a film of indium is formed on the collector. However, there has been such a problem that the amount of indium ion to be reduced on the collector is extremely small in the initial state of the contact between zinc and the collector so that hydrogen is reduced and hydrogen gas is generated.
An attempt has been made to suppress the corrosion and dissolution of zinc and the collector using a compound of metal nobler than zinc as the inhibitor. However, in the case of conventional zinc in which there are impurities such as iron, it is necessary to use a large amount of the inhibitor, and it has been impossible to suppress the corrosion and dissolution of zinc unless, for example, large amount of lead monoxide, which is a non-pharmaceutical harmful substance and may cause environmental pollution, is used. Further, there has also been such a problem that when large amounts of lead monoxide and an indium compound are added, needle-like crystals are deposited which break through a separator to cause a short circuit.
In recent applications, mercurated zinc has been used in a coin or button type silver oxide battery which prevents hydrogen gas generation and self-discharge of the battery.
In recent years, efforts have been made to improve the additives used in zinc powder, the separators, the sealing agents, the gelling agents and the elimination of mercury for cylindrical alkaline batteries. However, in the case of a coin or button-type silver oxide battery in which there is no escape for the hydrogen gas due to the structure of the battery, there are problems such as the occurrence of expansion and liquid leakage due to gas pressure, self-discharge of the battery and achieving the elimination of mercury.
The foregoing describes problems associated with conventional inhibitors and the amount of water in the battery. Although improvements with conventional inhibitors has resulted in improvements in the characteristics of the alkaline battery, it has not been possible to achieve the effective removal of mercury. Further, improvements thereof will be described hereinafter.
When only a conventional inorganic inhibitor is used, such problems arise in that the inhibitor is not homogeneously distributed on the collector because of a fairly small amount of the electrolyte solution in an actual battery and no metal coating is given, and bubbles generate between a negative electrode combined agent and the collector.
The manufacture of the collector using a metal such as indium or tin, or plating the collector with these metals is fairly effective in solving the above described problems.
However, when tin is used, although it is possible to suppress the hydrogen gas generation as compared with a case in which a collector of copper is used, no effect of a degree equivalent to the case of the use of mercury has been obtained.
When indium is used, although the effect is certainly larger than that of tin, there has been such a problem that the raw material is expensive, thus increasing the manufacturing cost. In the case of indium plating, there have been such problems that the application process is poor, no homogeneous film is provided, impurities remain on the surface, and the effect is weakened.
In addition, when no inhibitor is used and the collector is only coated with a metal, there has been such a problem that a countermeasure is insufficient for preventing hydrogen gas formation.
Further, it has been impossible to ensure sufficient prevention self-discharge even when fluorocarbon/polyoxyethylene series, polyoxyethylene alkylamide and the like are used which are organic inhibitors considered to be effective in the prior art.
Namely, it has been found that there is such a problem that no sufficient effect is obtained using each of the corrosion preventing methods in the prior art. As a result of reconsideration of the role of each inhibitor, it has been found that better effects are obtained by using a collector coated with zinc or a metal having a hydrogen overvoltage higher than that of zinc, together with the use of various inhibitors.
When a higher battery capacity is desired, a decrease in the battery capacity due to the absence of mercury must be compensated. In the case of a cylindrical alkaline dry battery, it is desired to increase the quantity of zinc powder as the active material. However, there has been such a problem that in the case of the button-type or coin-type alkaline battery in which there is no room from a viewpoint of space, such accomplishment is almost impossible.
On the other hand, in a clock or watch which uses an alkaline battery containing mercury, the battery containing mercury is recovered at retail stores in the case of a battery exchange. However, there has been such a problem that when the main body of the clock or watch is discarded, the mercury which is also discarded contributes to environmental pollution. Particularly with the continuing decrease in price of clocks and watches, there is the possibility that the number of clocks or watches to be discarded due to the service life expiration of the batteries increases.
An object of the present invention is to provide an alkaline battery which does not contain mercury.
Another object of the present invention is to provide an alkaline battery comprising mercuryless zinc powder as a negative electrode active material.
Still another object of the present invention is to provide an apparatus using an alkaline battery which does not contain mercury.
A further object of the present invention is to provide an apparatus using an alkaline battery comprising mercuryless zinc powder as a negative electrode active material.
When an indium compound such as indium sulfate, indium sulfamate, indium chloride or the like is added as an inhibitor into an electrolyte solution or a negative electrode active material, so as to allow indium ion to exist in the electrolyte solution in an amount sufficient to coat zinc and the collector, indium can be immediately deposited on zinc and the negative electrode collector. By coating the zinc and the collector with indium having a high hydrogen overvoltage, it is possible to prevent corrosion and dissolution of the two.
Further, in order to effectively utilize these inhibitors, a complexing agent is added into the electrolyte solution beforehand, and indium ion which is generated by dissolution of the indium compound is subjected to complexing. Thus it is possible to prevent hydrogen gas formation during deposition of indium and the decrease in the conductivity of the electrolyte solution due to precipitation of indium which is not deposited as a hydroxide.
The problem caused by the use of the inhibitor of only one species of the compound of metal can be solved by adding two or more species of compounds selected from an indium compound, a tin compound containing tetravalent tin and lead oxide into the electrolyte solution or the negative electrode active material, and depositing metal contained in these compounds as an allow onto zinc and the negative electrode collector. Characteristics, which cannot be obtained by a single metal coating, are obtained by depositing the two or more species of metals. electrode active material, so as to allow indium ion to exist in the electrolyte solution in an amount sufficient to coat zinc and the collector, indium can be immediately deposited on zinc and the negative electrode collector. By coating zinc and the collector with indium having a high hydrogen overvoltage, it is possible to prevent corrosion and dissolution of them.
Further, in order to effectively utilize these inhibitors, a complexing agent is added into the electrolyte solution beforehand, and indium ion which is generated by dissolution of the indium compound is subjected to complexing, thereby it is possible to solve the tasks of the hydrogen gas formation during deposition of indium and the decrease in the conductivity of the electrolyte solution due to precipitation of not deposited indium as hydroxide.
The problem caused by the use of the inhibitor of only one species of the compound of metal can be solved by adding two or more species of compounds selected from an indium compound, a tin compound containing tetravalent tin and lead oxide into the electrolyte solution or the negative electrode active material, and depositing metal contained in these compounds as an alloy onto zinc and the negative electrode collector. Characteristics, which cannot be obtained by single metal coating, are obtained by depositing the two or more species of metals.
When zinc in which indium is added in a relatively high concentration is used, it is considered that the corrosion and dissolution of zinc or the collector are suppressed owing to a mechanism that indium ion dissolved from zinc is deposited on the collector, and a film of indium is formed on the collector. However, in the initial stage of the contact between zinc and the collector, the amount of indium ion to be reduced on the collector is extremely small. Thus, when the one or more species of the compounds selected from the indium compound which is nobler than zinc, the tin compound containing tetravalent tin and lead oxide are added as the inhibitor into the electrolyte solution or the negative electrode active material, and the metal ion is allowed to exist in the electrolyte solution in an amount sufficient to coat zinc and the collector, then it is possible to immediately deposit lead onto zinc and the negative electrode collector, and it is possible to suppress the generation of hydrogen. In this case, depending on the amount of indium in the zinc powder it is preferable that the adding amount of the inhibitor is about 10-1000 ppm with respect to the zinc powder. If the adding amount is small, it,is impossible to sufficiently coat zinc and the collector, while if the adding amount is too large, there is such a bad effect that needle-like crystals penetrate through a separator to cause a short circuit.
When mercuryless zinc powder in which the content of iron is not more than 4 ppm with respect to a weight of zinc is used, one or more species of compounds selected from an indium compound nobler than zinc, a tin compound containing tetravalent tin and lead oxide are added as the inhibitor into the electrolyte solution or the negative electrode active material, and ion is allowed to exist in the electrolyte solution in an amount sufficient to coat zinc and the collector, then it is possible to immediately deposit a coating film of metal which is nobler than zinc on zinc and the negative electrode collector, and it is possible to suppress the corrosion and dissolution of zinc and the hydrogen generation accompanying therewith. In this case, it is preferable that the adding amount of the inhibitor be about 10-1000 ppm with respect to the zinc powder. If the adding amount too small, it is impossible to sufficiently coat zinc and the collector, and there is such a bad effect that needle-like crystals penetrate through a separator to cause a short circuit.
Using mercuryless zinc containing at least one species of metals of gallium, indium, lead, bismuth, aluminum, calcium and the like which are said to have an effect of increasing the hydrogen overvoltage and an effect of adjusting the particle shape during manufacturing of particles as the negative electrode active material, in the case of an electrolyte solution of the potassium hydroxide type, the amount of water in the battery is 0.31-0.57 mg (or 0.31-0.57 xcexcL at ordinary temperature) per a weight of 1 mg of mercuryless zinc, or in the case of an electrolyte solution of sodium hydroxide type, the amount of water in the battery is 0.32-0.59 mg (or 0.32-0.59 xcexcL at ordinary temperature) per a weight of 1 mg of mercuryless zinc, thereby it is possible to produce a coin or button type silver oxide battery in which the gas generation amount is not more than 0.03 xcexcL/g/day, and the self-discharge rate is not more than 4%/year.
Further, in order to approximate the performance of the battery containing mercury, it is necessary to combine and use various techniques.
Mercuryless zinc alloy powder added with metals of indium, lead, bismuth, calcium, aluminum and the like and a gelling agent for holding moisture in the battery are used as a negative electrode combined agent, a collector having the outermost layer which is coated with zinc or metal having a hydrogen overvoltage higher than that of zinc is used, and an inhibitor selected from an indium compound, lead oxide, hydroxide of alkaline earth metal, and a surfactant having polyoxyethylene group is added into the electrolyte solution or the negative electrode active material, thereby it is possible to obtain a battery in which the hydrogen gas generation is less, and the electric characteristics are good.
Especially, with respect to the coating of the collector, an alloy layer containing zinc as an essential element and containing one or more species selected from indium, lead and tin as a selective element is provided, thereby it is possible to provide an alkaline battery which is advantageous from a viewpoint of cost and in which the hydrogen gas generation can be suppressed.
Further, there are such problems due to the elimination of mercury that the self-discharge increases and the capacity decreases. However, since the zinc alloy layer of the collector is made thick to some extent, it is possible to increase the capacity of the battery without significantly changing the spacing in a battery casing.
Indium compounds such as indium sulfate, indium sulfamate, indium chloride and the like dissolve in a concentrated caustic alkali solution, which forms alkaline complex ion capable of cathode reduction referred to in the plating.
The alkaline complex ion of indium is subjected to zinc surface reduction representing a potential lower than a reduction potential of itself, and indium is immediately deposited as metal. In addition, the collector comprised of a material such as copper or the like contacts with zinc, so that the same potential as that of zinc is obtained, and indium is deposited in the same manner. When the surfaces of zinc and the collector are initially coated with indium, all of the surfaces achieve the same potential of indium, and the electrochemical driving force is lost, so that further deposition of indium is ceased. However, when the zinc surface is newly exposed by discharge, indium which exists as the alkaline complex ion is immediately reduced and deposited.
The inhibitor of the indium compound functions more effectively by adding the complexing agent. This is due to the fact that the alkaline complex ion of indium and hydrated indium ion are unstable, so that they precipitate with no complexing agent to be insoluble in the electrolyte solution, or even if they are dissolved, they precipitate as hydroxide due to minute environmental changes, or they are apt to change into a viscous solution such as polyion (those similar to the description in Inorganic Chemistry Series 7, Coordinate Stereochemistry, written by Yoichi NIIMURA, published by Baihukan Co. Ltd., 65-66). In addition, the alkaline complex ion of indium and the hydrated indium ion are unstable and have low deposition potentials, but they have wide ranges of the potential, and the hydrogen gas generation is also induced during the deposition. Thus, when the complexing is performed with tartarate or EDTA, a stable complex ion is provided; it is possible to narrow the range of the deposition potential of indium, it is possible to separate from the deposition potential of hydrogen, and it is possible to deposit only indium without the accompanying generation of hydrogen.
Other than indium, tin and lead also have smaller ionization tendencies than zinc, so that when ions of these metals are allowed to exist in the electrolyte solution, it is possible to deposit these metals on the zinc surface. In addition, because the collector contacts with zinc, the same potential as that of zinc is obtained, and the above-mentioned metal is deposited. The function for further suppression of the corrosion and dissolution by mixing the indium compound, the tin compound containing tetravalent tin, lead oxide and the like is not certain, however, the following facts can be considered.
One is the fact that alloy formation may take place when these metals are deposited on zinc and the collector. During the alloy formation, fine crystals of the deposited metal are produced, and the surfaces of zinc and the collector are coated with homogeneous films having no deficiency. For example, in the case of an alloy of indium-tin, fine crystals are produced when a composition near the eutectic point of about 50:50 in an atomic % is aimed. In the case of an alloy of the three component system, crystals become more complicated, and coarse formation of crystal grains is prevented, so that it becomes easy to obtain a homogeneous film.
The other is the fact that characteristics possessed by each of the metals can be simultaneously utilized by mixing. Particularly in the case of lead, if it is used alone needle-like crystals are deposited and it becomes impossible to homogeneously coat the surfaces of zinc and the collector. However, during assembly of the battery, when the electrolyte solution containing tetravalent tin is first added and subsequently the electrolyte solution containing lead oxide is added, the zinc and the collector are coated with indium and tin which form relatively homogeneous films and then are coated with a film having needle-like crystals of lead. The homogeneous films of indium and tin suppress the corrosion and dissolution of zinc or the collector, and the film of lead having needle-like crystals strengthens the electrical contact between the zinc and the collector, and enhances shock resistance and discharge characteristics.
In the case of a negative electrode active material such as pure zinc and the like in which the hydrogen generation is large, it is difficult to form a homogeneous film due to the hydrogen generation which occurs as a competitive reaction with respect to the deposition of metal on the surface, and the effect of deposited metal becomes less. Thus, using a negative electrode active material in which zinc is added with indium, bismuth, lead, aluminum, calcium, gallium and the like, effectively suppresses the generation of hydrogen to some extent.
Attempts have been made to add zinc with a metal having a high hydrogen overvoltage to form an alloy so as to suppress corrosion and dissolution. It is known that the effect thereof is remarkably expressed especially when indium is added. When zinc is used in which indium is added in a relatively high concentration, for example, not less than 400 ppm, a part of indium and zinc is once dissolved due to the contact with copper or the like of the collector. It is considered that the corrosion and dissolution of zinc is suppressed by a mechanism in which dissolved indium ion is deposited on the collector, and a film of indium is formed on the collector. However, in the initial stage of the contact between zinc and the collector, the amount of indium ion to be reduced on the collector is extremely small. Thus, when one or more species of the compounds selected from the indium compound which is nobler than zinc, the tin compound containing tetravalent tin and lead oxide are added as the inhibitor into the electrolyte solution or the negative electrode active material, and the metal ion is allowed to exist in the electrolyte solution in an amount sufficient to coat zinc and the collector, then it is possible to immediately deposit lead onto zinc and the negative electrode collector, and it is possible to suppress the hydrogen generation.
When iron is abundant in zinc, there are provided many places in which iron is exposed on the zinc surface. Zinc and iron on the surface form a local battery in the electrolyte solution, zinc dissolves, and hydrogen is generated from iron. At the place in which hydrogen is generated, the coating film by the inhibitor is difficult to be formed, and it is difficult to obtain the effect of the inhibitor.
When mercuryless zinc powder in which the content of iron is not more than 4 ppm with respect to a weight of zinc is used, one or more species of the compounds selected from the indium compound nobler than zinc, the tin compound containing tetravalent tin and lead oxide are added as the inhibitor into the electrolyte solution or the negative electrode active material, and the ion is allowed to exist in the electrolyte solution in an amount sufficient to coat zinc and the collector, then it is possible to immediately deposit the film of metal nobler than zinc onto zinc and the negative electrode active material, and it is possible to suppress the corrosion and dissolution of zinc and the hydrogen generation accompanying therewith.
It is preferable that the adding amount of the inhibitor be about 10-1000 ppm with respect to the zinc powder. If the adding amount is less, it is impossible to sufficiently coat zinc and the collector, and there is such a bad effect that needle-like crystals penetrate through the separator to cause a short circuit.
It is generally said that gallium, indium, lead and bismuth have high hydrogen overvoltages, and when they are added to zinc, the hydrogen gas generation is suppressed. It is said that aluminum and calcium smoothen the surface during the production of zinc powder by atomization, reduce the surface area of the zinc powder, and suppress the hydrogen gas generation in the same manner.
In addition, although the function is not certain, the electrolyte solution is made as much as permitted by liquid leakage, thereby it is possible to suppress the hydrogen gas generation and the self-discharge. Further, by increasing the electrolyte solution, it also becomes possible to increase the absolute amount of the inhibitor to be added in order to suppress the hydrogen gas generation and the self-discharge, and it is considered that the inhibitor effect is also increased.
In order to allow the mercuryless battery to approximate the performance of a battery in which mercurated zinc is contained, it is necessary that features of various techniques are understood, and that they are combined and used. Functions of individual techniques will be shown hereinafter.
Generally speaking, the addition of metals such as indium, lead, bismuth, calcium, aluminum and the like into zinc is to prevent the corrosion and dissolution and suppress the hydrogen gas generation. However, the role of each metal added varies. The metal having a high hydrogen overvoltage such as indium, lead and bismuth forms an alloy with zinc, increases the hydrogen overvoltage, and prevents the corrosion and dissolution. On the other hand, calcium and aluminum smoothen the alloy surface during manufacturing by atomization, which level potential distribution, and decrease the surface area of zinc particles, so that they are effective in the prevention of the corrosion and dissolution. In addition, when hydrogen is generated on the zinc surface by the corrosion and dissolution, no inhibitor is supplied there and no effect of the inhibitor can be also exhibited. Also in this meaning, the alloy formation of zinc by the addition of metals according to the present invention is important.
The cross-linked type polyacrylic water-absorbing polymer to be used as the gelling agent has a strong moisture holding property, which prevents the electrolyte solution from evaporation and unnecessary movement toward the separator and the positive electrode side. Thus, it is possible to allow the inhibitor to be distributed all over and suppress the increase in the internal resistance due to shortage of liquid at the last stage of discharge.
The reason why the layer of zinc or metal having a hydrogen overvoltage higher than that of zinc is formed at the outermost layer of the collector is to prevent the fact that the mercuryless zinc powder as the negative electrode active material contacts with copper of the collector to form a battery, and hydrogen gas is also generated from it.
In addition, when the layer of zinc or metal having a hydrogen overvoltage higher than that of zinc is formed after the press processing of a negative electrode casing containing the collector, it is possible to shield impurities such as iron and the like adhered to the collector side during the processing.
Especially, it is effective to provide the alloy layer containing zinc as the essential element and containing one or more species selected from indium, lead and tin as the selective element on the collector surface. This is due to the fact that the potential of the collector surface becomes substantially the same as that of the mercuryless zinc surface as the negative electrode active material. Thus, even when the collector contacts with the mercuryless zinc powder as the negative electrode active material, it is avoided that a local battery is formed and hydrogen gas is generated from it.
It is supposed that, even if only zinc is plated on the collector, the potential difference may disappear and an effect may be expected, however, zinc deposited by plating or the like is extremely active and easy to be corroded, so that little effect is provided for suppressing the hydrogen gas formation. Thus, it is necessary to plate an alloy in which indium and lead are added into zinc by not less than several 10 ppm. In the plating process, for the plating solution, several 10 to several 1000 ppm of the indium compound, lead compound and tin compound are added into general zinc plating solutions may be needed, and for the positive electrode, an alloy of zinc may be used. For example, indium sulfate may be added in the case of a plating solution of the zinc sulfate type, while indium cyanide may be added in the case of a plating solution of the cyanide type.
As the inhibitor, there are the inorganic type and the organic type. Among them, the inorganic inhibitor is roughly classified into two species. One is a compound of metal which is nobler than zinc and has a higher hydrogen overvoltage. For example, it is the above-mentioned indium compound such as indium sulfate, indium sulfamate, indium chloride, indium hydroxide or the like, and they are dissolved in a concentrated caustic alkaline solution, and form alkaline complex ion capable of cathode reduction referred to in the plating. The alkaline complexion of indium is reduced at the zinc surface representing a potential lower than the reduction potential of itself, and indium is immediately deposited as metal. When zinc is initially coated with indium, all of the surface becomes to have the potential of indium, and the electrochemical driving force is lost, so that further deposition of indium is ceased. However, when the zinc surface is newly exposed by discharge, indium which exists as the alkaline complex ion is immediately reduced and deposited. Thus, there are provided such effects that the hydrogen gas generation is less even when the discharge is stopped on the way to store, and the self-discharge rate is made small.
Due to the fact that a potential lower than the reduction potential of itself is indicated also on the collector such as copper or the like contacting with zinc, the alkaline complex ion of indium is reduced, and indium is immediately deposited as for metal. However, the amount of the electrolyte solution is fairly less in an actual battery, so that the inhibitor is not homogeneously distributed on the collector to provide no metal coating, and when there are bubbles between the negative electrode combined agent and the collector, such a place only where no metal coating is provided at all is made, and it has been impossible to exhibit a sufficient effect. Lead monoxide and the tetravalent tin compound are included in the inorganic inhibitor of this type.
The other inorganic inhibitor is oxide and hydroxide of metal baser than zinc or non-metal. Although the function is not certain, effects are provided in the suppression of the hydrogen gas generation and the improvement in electric characteristics, and as the representative one, there is barium hydroxide or the like which is hydroxide of alkaline earth metal. However, when a collector which having no layer of zinc or metal having a hydrogen overvoltage higher than that of zinc for t h e outermost layer is used, the hydrogen gas generation due to the contact between the collector and zinc is too large, and little effect thereof is expressed.
The surfactant of organic type suppresses the corrosion and dissolution of zinc because the hydrophilic group adheres to the zinc surface a nd the hydrophobic group suppresses the approach of water and hydroxyl group to the surface. The effect is similar to that of a hydroxide of alkaline metal, and it is desirable to combine and use with the collector having the layer of zinc or metal having a hydrogen overvoltage higher than that of zinc for the outermost layer.
In addition, although the function is not certain, when polyoxyethylene alkylamide, barium hydroxide and the like are combined and used, a remarkable hydrogen gas generation suppressing effect is obtained. However, organic surfactants have a possibility to be excessively adhered onto the surfaces of zinc and the collector and inhibit the reduction on the surface in the combination with the indium compound or lead monoxide, so that it is desirable that the adding amount is made as small as possible within a range which can provide the effect.
It has been found out that there are roles of each one such that the coating of zinc or metal having a hydrogen overvoltage higher than that of zinc onto the collector suppresses the hydrogen gas generation due to the contact between zinc and the collector, the indium compound and lead monoxide suppress the self-discharge after partial discharge, and the alkaline earth hydroxide improves electric characteristics, and the maximum effect is obtained when they are combined and used. Namely, when the alkaline battery is manufactured by using the collector having the layer of zinc or metal having a hydrogen overvoltage higher than that of zinc for the outermost layer, it is possible to obtain the one in which the self-discharge rate is small before use and after partial discharge, and the electric characteristic is good.
When the collector is coated with a zinc alloy, the zinc alloy becomes the negative electrode active material as it is, so that by thickening the zinc alloy layer on the collector surface, it is possible to make up a battery capacity reduced by elimination of mercury. For example, when a collector having a diameter of 6 mm is plated by 10 xcexcm, the amount of the negative electrode active material increases by 2 mg because the specific gravity of zinc is 7.13. Provided that the amount of the negative electrode active material as zinc powder is 30 mg, it is possible to increase the capacity by about 6.7% without changing the space in the battery can so much. However, as compared with the powdery negative electrode active material, the surface area is small, so that no large current can be expected. However, it is most suitable for increasing the capacity of a batteries which perform minute discharge such as batteries for clocks and watches.
In addition, to thicken the zinc alloy plating is advantageous also in the processing of battery cans. Generally, the negative electrode can having the negative electrode collector is manufactured by punching a hoop material in many cases. In this case, if the zinc alloy has plated rather thickly beforehand at the side of hoop material for serving as the collector, the probability to expose metal such as for example copper as a base substrate during processing is reduced.